The development of effective vaccines for intracellular microbial pathogens, such as mycobacteria, toxoplasma, plasmodium, and leishmania, remains an elusive goal. Despite substantial efforts to define the mechanisms required for resistance, develop new adjuvants, and identify protective antigens, the long-lived cellular immunity that can be generated in response to infection is not always recapitulated by vaccination. For example, in spite of the strong immunity that develops following an infection with leishmania, efforts to develop a vaccine for human leishmaniasis have been unsuccessful. Lack of success is due in part to the limited understanding of the T cells that mediate protection. While we have identified circulating T cell subsets that contribute to immunity in leishmaniasis (1), these circulating T cells fail to provide the level of immunity observed in mice that have resolved a primary infection with L. major. Using a combination of skin grafting and adoptive transfers, however, we found that leishmania-responsive IFN-? producing CD4+ T cells resident in the skin are the missing link required for optimal protective immunity (2). We propose to identify how they are generated and maintained in the skin, determine how they promote immunity, and target them in a leishmanial vaccine. These studies are important for leishmaniasis, and have broad implications for vaccine development against other pathogens. In Aim 1 we will define the factors that regulate the accumulation of CD4+ resident memory T cells in the skin following resolution of a primary infection. These studies have direct relevance to vaccine development, as they will define what will be required for generating resident memory T cells. In Aim 2, we focus on the mechanisms by which resident memory T cells mediate protection. Here we will identify the effector T cells that synergize with resident memory T cells to mediate protection, and determine if resident memory T cells help to amplify the generation of T effector cells. Finally, in Aim 3 we will translate this information into an experimental vaccine. We will create a recombinant vaccinia virus expressing a leishmanial antigen, and assess the protection induced by this vaccine and qualitatively and quantitatively define the T cells that contribute to that protection.